|
ANTENNA
/ RADIO SWITCH - MK2
RF
3 position switch for either antenna switching or transceiver
switching 0 - 150MHz.
In order to connect multiple radios operating within the 144MHz - 148MHz band to a single antenna, a switch was needed. Upon evaluating the MK1 version of my antenna/radio switch, it was discovered that it barely met the port isolation requirement for the specified frequency range. This limitation resulted in the transmitter power being restricted to less than 25 Watts and posed a risk of inadvertently damaging the receiver of one of the radios. Consequently, a redesign of the radio switch became necessary.
Photo
1
In service 3 radio ports to single antenna port
switch.
In
order to address the need for improved switch port isolation, the RF
switch must be designed to ensure effective isolation between the
ports. While port isolation may be less critical when switching
between antennas, it becomes crucial when switching between radios
that transmit from one port, as the ports connected to receivers,
even when turned off, require adequate isolation to prevent receiver
front end damage.
Considering
these requirements, the specifications for the switch are as
follows:
Maximum
RF power: 100 Watts (50 dBm)
Maximum
frequency: 150 MHz
Minimum
port isolation: -45 dB
Additionally,
the switch design should incorporate a 12V DC switched circuit to
accommodate a masthead pre-amplifier.
Assumptions
The
above specification are based on Ian (W2AEW)’s presentation on
issues relating to using an antenna switch to switch radio
transceivers.
Refer
to video link: https://www.youtube.com/watch?v=lMMql1gEORQ
The
video suggests that many ham radio transceiver receivers are capable
of withstanding up to +20dB (100mW), however Ian (W2AEW) decided
that as a matter of caution he would base his calculation on +10dB
(10mW). Even +10dB may be way too much for many receivers such as
scanners etc., therefore study equipment specification to determine
suitability.
Radio
specification examples
·
The
Icom IC-706MK2G transceiver’s service manual on page 1 states; DO
NOT apply an RF signal of more than 20 dBm (100 mW) to the antenna
connector. This could damage the transceiver’s front end.
·
The RSP1 SDR radio states on page 5 of the manual that; In
any configuration the maximum input power to the RSP1 must not
exceed 0dBm.
·
The maximum RX power of HackRF One is -5 dBm. Exceeding -5
dBm can result in permanent damage!
Calculations
A
100W transmitter would represent 50dBm
Therefore
50dBm (100W) – 10dB (safe receiver exposure) = 40dB (minimum
required port isolation)
Based
on the above calculation the RF switch just satisfies the required
port isolation at the desired 146MHz (2m band), but would not be
suitable at higher power at the 2m band and would not be suitable at
higher frequencies such as the 70cm band at almost any realistic
power.
Below
54MHz sufficient port isolation would allow for 200W transmitter to
be used however the current rating of the relays used is 1 Amp per
pole and are connected with two poles in parallel producing a rating
of up to 2 Amps and would restrict power to 100W.
100W
of RF at 50Ohms impedance will produce a voltage of just over 70V at
current of just over 1.4Amps therefore the radio switch is really
only suitable for power levels at or below 100W.
Design
Fig 1
Schematic of the radio switch unit
Construction
The switch is a simple 3 position single pole wafer switch that activates
a relay for each radio input. When power up only one relay can be
activated at a time with the other relays switched to ground and
when the unit is not powered up all relays will switch to ground to offer the
radios some protection from the effects of lightning.
The
relays are mounted to a grounded PCB with grounded PCB shield
mounted between the relays. All internal coax interconnections are
with RG316/U
Photo
2 Relays mounted with grounded shielding.
C1,
C2 and C3 are to ground any RF and prevent RF from travelling out on
the DC supply.
The 12DC switched circuit for a masthead pre-amp presents a DC supply to
a 2.1mm DC panel socket.
Testing
Test was carried out using a NanoVNA calibrated for Mag with the switch
switched to an adjacent port with a 50ohm load connected. The below
results and chart show port isolation against various frequencies.
Photo
3 Test set up for port isolation evaluation.
|
Frequency
MHz
|
Port
Isolation dB
|
|
1.000
|
-78
|
|
3.500
|
-77
|
|
10.000
|
-75
|
|
15.000
|
-73
|
|
30.000
|
-63
|
|
54.000
|
-56
|
|
146.000
|
-45
|
|
435.000
|
-19
|
TOP
OF PAGE
Page initiated 28 April,
2023
Page
last revised 23
September, 2023
|
